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Purpose: In treatment planning of charged-particle radiotherapy, patientheterogeneity is conventionally modeled as variable-density water convertedfrom CT images to best reproduce the stopping power, which may lead toinaccuracies in the handling of multiple scattering and nuclear interactions.Although similar conversions can be defined for these individual interactions,they would be valid only for specific CT systems and would require additionaltasks for clinical application. This study aims to improve the practicality ofthe interaction-specific heterogeneity correction. Methods: We calculated theelectron densities and effective densities for stopping power, multiplescattering, and nuclear interactions of protons and ions, using the standardelemental-composition data for body tissues to construct the invariantconversion functions. We also simulated a proton beam in a lung-like geometryand a carbon-ion beam in a prostate-like geometry to demonstrate the procedureand the effects of the interaction-specific heterogeneity correction. Results:Strong correlations were observed between the electron density and therespective effective densities, with which we formulated polyline conversionfunctions. Their effects amounted to 10% differences in multiple-scatteringangle and nuclear-interaction mean free path for bones compared to those in theconventional heterogeneity correction. Although their realistic effect onpatient dose distributions would be generally small, it could be at the levelof a few percent when a beam traverses a large bone. Conclusions: The presentconversion functions are invariant and may be incorporated in treatmentplanning systems with a common function relating CT number to electron density.This will enable improved beam dose calculation while minimizing initial setupand quality management of the user's specific system.

Relationship between electron density and effective densities of body  tissues for stopping, scattering, and nuclear interaction of proton and ion  beams